Spacewalkers take 2nd shot at ISS power fix

CAPE CANAVERAL -- Armed with a red toothbrush, a wire brush and grease, two spacewalking astronauts will make another attempt today to re-boost electrical power at the International Space Station.

Sunita "Suni" Williams and Akihiko Hoshide will attempt to bolt a spare power-switching unit in place on the station's central truss - a task that gave them repeated trouble last week.

Doing so would restore two of eight power channels that route electricity from the station's massive American solar arrays to critical outpost systems and laboratory facilities.

Another channel was lost when a voltage regulator coincidentally failed over the weekend.

"So out of the eight channels of power on the International Space Station, from those big giant solar arrays, three of them are suffering some challenges right now," NASA mission commentator John Byerly said Tuesday.

Williams, an American flight engineer, and Hoshide of Japan Aerospace Exploration Agency are scheduled to exit the station's U.S. Quest airlock at 7:15 a.m. EDT. It's their second spacewalk in six days.

They'll work to clean and lubricate two bolts and stanchions designed to secure the switching unit to a cold plate on the truss.

During an eight-hour, 17-minute spacewalk last Thursday, the astronauts were unable to adequately tighten one of the bolts. They were only able to get nine of 15 turns required to secure the unit and provide electrical connectivity with the station's American power grid.

Several attempts were made, but the bolt kept locking up. The astronauts detected metal shavings in the area. Engineers think metal flakes could be stuck in the threads of the stanchion.

A timeline beamed up to the crew Tuesday shows the astronauts will have a limited amount of time to install the spare switching unit.

It shows a "bingo time" four hours into the excursion. If the job is not done by then, the astronauts will be told to clean up their work site and return to the airlock with the switching unit. In that case, the switching unit would be taken inside the outpost for examination.

The switching unit is one of four that route electricity from the station's lengthy solar arrays to outpost systems and lab facilities.

The four solar arrays each have two panels that feed electricity through a specific switching unit. So altogether, there are eight channels in the U.S. power grid.